1. Field of the Invention
The present invention relates to a high-pressure discharge lamp, and more particularly to a long-life high-pressure discharge lamp which is arranged to prevent the internal gases from leaking out and also to prevent the lamp bulb from being ruptured, and which suffers reduced blackening and luminance drop even after it has been turned on over a long period of time.
2. Description of the Related Art
As shown in FIG. 1 of the accompanying drawings, high-pressure discharge lamp 101 generally has a pair of confronting tungsten electrodes 102 inserted in lamp bulb 101 of quartz glass which has a spherical central portion. Electrodes 102 are inserted respectively from insertion slots 104 defined in the respective opposite ends of lamp bulb 101. Insertion slots 104 are hermetically sealed by respective electrodes 102 fitted in respective sleeves 105 of molybdenum foil which serve as thermal dampers. Mercury, a halogen gas such as of methylene bromide or the like, and an inactive gas such as of argon or the like are introduced and confined in lamp bulb 101.
For example, mercury is introduced and confined at a rate of 0.15 mg/mm3 or higher in the lamp bulb 101. When a trigger voltage is applied between electrodes 102, a glow discharge is induced between electrodes 102 in the presence of the inactive gas, vaporizing the mercury, and a plasma discharge caused in the high-pressure mercury gas radiates highly color rendering light with high luminance. Since high-pressure discharge lamps are capable of emitting highly color rendering light with high luminance, they have in recent years attracted much attention and widely been used as light sources for projection-type liquid crystal display devices or the like.
It has been pointed out that early high-pressure discharge lamps suffer blackening on the inner wall surfaces of their lamp bulbs, resulting in a reduction in the amount of emitted light, after they have been turned on over a long period of time. Such a phenomenon is caused when tungsten W vaporized from electrodes 102 by a discharge at high temperature are deposited on the bulb wall, as shown in FIG. 1. In an effort to prevent blackening on the bulb wall halogen gas has been sealed in the lamp bulb. The introduced halogen gas generates halogen ions at the high temperature, and the halogen ions are united with the tungsten deposited on the bulb wall and vaporized and deposited on electrode bases which are of a relatively low temperature. Such a halogen cycle is repeated to prevent the blackening of the bulb wall.
The halogen gas usually comprises a halogen compound such as methylene bromide or the like. When the discharge lamp is energized, the halogen compound is decomposed, generating halogen ions. The halogen gas is introduced and confined in the lamp bulb in an amount that is effective to prevent the blackening, i.e., in such an amount that the halogen sealed in the lamp bulb has a partial pressure of 1×10−6 μmol/mm3 or higher.
An inactive gas such as of argon or the like is also introduced and confined in the lamp bulb under a pressure ranging from 6×103 Pa to 6×104 Pa in order to induce a glow discharge when the discharge lamp is turned on.
If the halogen gas introduced and confined in the high-pressure discharge lamp exists in an excessive amount, then it tends to erode and degrade electrodes 102 and molybdenum foil sleeves 105 at the sealed ends of the lamp bulb. When electrodes 102 and molybdenum foil sleeves 105 are highly eroded and degraded, since a high pressure of 100 atms is developed in the lamp bulb due to the vapor pressure of the sealed mercury, the gases tend to leak from the sealed ends of the lamp bulb, and the sealed ends are likely to be ruptured. In order to prevent the bulb wall from blackening, to prevent the gases from leaking, and also to prevent the lamp bulb from being broken, research efforts have been made to improve high-pressure discharge lamps, including designing the structure thereof and adjusting the amounts of various components of gases to be sealed in the lamp bulb.
For example, Japanese laid-open patent publication No. 11-149899 discloses a high-pressure discharge lamp which is filled with mercury in an amount ranging from 0.12 to 0.35 mg/mm3 and a halogen gas in an amount ranging from 10−7 to 10−2 μmol/mm3, with the electrodes containing 12 ppm of potassium oxide.
Japanese patent No. 2829339 reveals a high-pressure discharge lamp which contains mercury in an amount ranging from 0.2 to 0.35 mg/mm3 and a halogen gas in an amount ranging from 10−6 to 10−4 μmol/mm3.
Japanese patent No. 2980882 reveals a high-pressure discharge lamp which contains with mercury in an amount ranging from 0.16 mg/mm3 or higher and a halogen gas in an amount ranging from 2×10−4 to 7×10−3 μmol/mm3. Preferably, the load on the bulb wall is 0.8 W/mm2 or higher, and an inactive gas is introduced and confined in the lamp bulb at a pressure of 5×103 Pa or higher.
Japanese laid-open patent publication No. 11-297274 discloses a high-pressure discharge lamp which contains mercury in such an amount that it develops a vapor pressure ranging from 100 to 200 atms at the time the high-pressure discharge lamp is turned on, and a halogen gas in an amount ranging from 1.1×10−5 to 1.2×10−7 mol/cc.
However, the problems of the reduction in luminance due to blackening, leakage of gases from the sealed ends, and rupture of the lamp bulb cannot be solved together no matter how the amounts of gases introduced and confined in the lamp bulbs are adjusted as disclosed in the above prior art.